The White House is debating, again, troop levels for Afghanistan. This time around, the question is, what happens after this year when the combat mission ends? The Pentagon wants to keep 10,000 troops there. That number would be enough to train Afghan forces and conduct counterterrorism raids, the Pentagon says. Whether the Pentagon gets what it wants is far from certain.

NPR's Tom Bowman has been reporting on this and he joins us now. And, Tom, for context, there are about 37,000 U.S. troops in Afghanistan now. The Pentagon has come up with this number of 10,000. Why that number, a dozen years after the war started?

TOM BOWMAN, BYLINE: Well, because, Melissa, there are two missions here. The main one is training Afghan forces at everything from how to fly helicopters, how to support themselves in the field, planning an operation. And then the other mission involves having U.S. special operations forces teaming up with Afghan forces to go after Taliban leadership and remnants of al-Qaida. There are hidden numbers here because all the soldiers needed to make that work, the support troops, will be at multiple bases around the country.

So you're looking at security at the bases, intelligence analysts, medevac, helicopter pilots. The numbers start to add up. Now, both the Wall Street Journal and The New York Times reported that 10,000 figure. We've confirmed it. And it's roughly the same number we've been hearing now for months.

BLOCK: But there are also reports, Tom, that the Pentagon has framed this as an all-or-nothing option, right, either 10,000 troops in Afghanistan or no troops. Is that right?

BOWMAN: Right. We've not confirmed that. And the - what they're saying is the idea of anything less than 10,000 just isn't sustainable. You can't do those two missions with less than 10,000. And what I have been hearing in my reporting, both in Afghanistan and Washington, is the military is basically saying to the White House and the Congress, listen, if you want us to do these two missions, we need that number of soldiers.

And people have told me that, listen, if you come up with several thousand U.S. troops in Afghanistan, they're really not going to be able to get out and do their job, do any mission. They'll be holed up in their bases just trying to be safe. But the White House says, listen, it's all being discussed. The president hasn't reached any decision yet.

BLOCK: And assuming U.S. troops do remain in Afghanistan, how long would the longer term mission last?

BOWMAN: Well, there have been some reports it would be a couple of years. By the end of President Obama's second term, all troops would be out. But the security agreement signed between - well, agreed between Afghanistan...

BLOCK: Not signed is the problem, right?

BOWMAN: ...and the U.S. - and not signed yet by President Hamid Karzai says the timeline is 2024. Nobody thinks that U.S. troops will be on the ground in Afghanistan that long. But that's what we're talking about here. And again, it's still being debated here. And also, this sounds a lot like the debate now, what Robert Gates, the former defense secretary, talked about in his book. On the one hand, you have the Pentagon saying we need X number of troops, and then you have people in the White House, particularly Vice President Biden saying, that's too many. Let's negotiate. And I think that's what's going on now.

BLOCK: Tom, we talked about a training mission and, of course, the U.S. has been training Afghan forces for years. The question over many years now has been why aren't the Afghan forces ready to handle their own country's security?

BOWMAN: Well, they are taking the brunt of the fight. Their casualty rates are vey high at a time when the U.S. casualty rates have dropped dramatically. And they haven't lost much ground to the Taliban. But they still need training and assistance. That could take some time, U.S. officials are saying. But, you know, political people in Washington are saying, listen, President Hamid Karzai is not a good partner. There's still rampant corruption. Nothing's really changed, so why have that many troops there in the end?

BLOCK: One last thing to bring up here, Tom. There are some members of Congress, notably Republicans John McCain and Lindsey Graham, who are pointing to Iraq and the resurgent to Iraq and the resurgence of al-Qaida there and saying, look, this is what happens when U.S. troops withdraw. Are they making that same case about Afghanistan?

BOWMAN: Well, in both countries, the U.S. has spent years and billions of dollars in building up local forces. But in Iraq, it's a different problem. It's largely a political problem that the Iraqi government hasn't dealt with. That spurred the fighting there. In Afghanistan, it's different. You don't have troops that are as well-trained as they were in Iraq. You have safe havens in Pakistan. So the sense is if you don't have troops, that place could fall much faster than we're seeing in Iraq.